Japanese Kokai Patent Application No. 2004-264126 discloses a conventional device for estimating the inputtable/outputtable power of a battery. In that device, the output current I and terminal voltage V of the secondary battery are detected. The current I and terminal voltage V are input to an adaptive digital filter using the battery model to estimate the parameters used in the mathematical formula of the battery model. The estimated parameters and current I and terminal voltage V are used to calculate open-circuit V0. The inputtable/outputtable power (i.e., possible charge/discharge power) estimating device estimates the inputtable power (possible charge power) of the secondary battery on the basis of the estimated parameters, open-circuit voltage V0 and upper limit voltage VMAX and estimates the outputtable power (possible discharge power) of the secondary power on the basis of the estimated parameters, open-circuit voltage V0 and lower limit voltage VMIN.